Matching of the respective impedances of a radio-frequency accelerating system (hereinafter referred to as a RF accelerating system") and a power supply is essential to the efficient acceleration of a charged particle beam. Continuous monitoring of the accelerating gap voltage is indispensable to the stable acceleration of a charged particle beam. In a conventional RF accelerating system, an impedance adjusting circuit element and an accelerating gap voltage measuring circuit element are connected directly to an accelerating gap 2 as shown in FIG. 8(a).
FIG. 8(a) shows a tuned RF accelerating system by way of example. The revolving frequency of a charged particle beam circulating through a ring type accelerator increases gradually as the charged particle beam is accelerated by a RF accelerator 1. The frequency of power supplied from a RF power supply 8 is varied in synchronism with the increasing revolving frequency of the charged particle beam to achieve a stable acceleration of the charged particles of the charged particle beam, thereby to raise the level of energy of the charged particle beam to an ultimate level of energy. In the tuned RF accelerating system, a capacitor 15 having an appropriate capacitance is connected across an accelerating gap 2 having an inner conductor 3 on opposite sides thereof, and the magnetic permeability of a magnetic core 4 is varied so that the resonant frequency of the RF accelerator 1 coincides with the frequency of the power supplied by the RF power supply 8 corresponding to the revolving frequency of the charged particle beam to accelerate the charged particle beam efficiently.
FIG. 8(b) shows an untuned RF accelerating system by way of example. The frequency of power supplied from a RF power supply 8 is varied in synchronism with the revolving frequency of a charged particle beam to achieve stable acceleration of the charged particle beam. In the untuned RF accelerating system, a resistor 6 is connected across an accelerating gap 2 to reduce the acuteness of resonance (Q value) of a RF accelerator 1 in order that a voltage at frequencies in a wide frequency range can be generated in the accelerating gap 2. Therefore, the untuned RF accelerating system does not need any control operation for making the resonant frequency of the RF accelerator 1 coincide with the frequency of the power supplied from the RF power supply 8 corresponding to the revolving frequency of the charged particle beam.
FIG. 8(c) shows a RF accelerating system in which a capacitor voltage divider 7 is connected across an accelerating gap 2 to measure accelerating gap voltage, i.e., voltage across the accelerating gap 2.
These prior art RF accelerating systems are mentioned in "OHO '89 Ko-Enerugi Kasokuki Semina", Chapter 5, "Yoshi Shinkurotoron no Ko-shuha Kasoku Sochi", pp. 19-32, Ko-Enerugi Kasokuki Kagaku Kenkyu Shorei-kai, and "Conceptual Design of a Proton Therapy Synchrotron for Loma Linda University Medical Center", pp. 25-27, Fermi National Accelerator Laboratory, 1986.
In the conventional RF accelerating system, the circuit element for impedance adjustment and the circuit element for accelerating gap voltage measurement are connected directly to the accelerating gap. Since the voltage across the accelerating gap for accelerating the charged particle beam is high, these circuit elements need to have a high withstand voltage. In particular, the capacitor among those circuit elements needs to be a large vacuum capacitor having a high withstand voltage. When such a large vacuum capacitor is connected across the accelerating gap, the structure around the accelerating gap becomes large and complicated, and such a large, complicated structure is difficult to assemble and disassemble.
A RF resistor having a high withstand voltage dissipates much power and is difficult to manufacture and hence it is difficult to generate a high accelerating gap voltage in an untuned RF accelerating system.